Investments - Narrative (Details)
$ in Thousands
3 Months Ended
Mar. 31, 2015
USD ($)
Property
State
Mar. 31, 2014
USD ($)
Dec. 31, 2014
USD ($)
Property
Investment [Line Items]      
Gross investment in real estate properties $ 8,224,762   $ 8,043,497
Number of real estate properties | Property 2,547   2,509
Number of properties securing borrowings | Property 145    
Properties dispersed geographically | State 49    
Minimum percentage of investment in real estate properties 10.00%    
Amortization amount to rental revenue for capitalized leases $ 1,400 $ 1,300  
Texas      
Investment [Line Items]      
Number of states exceeding disclosure threshold | State 1    
Percentage of investment in real estate properties 11.90%    
In-Place Leases      
Investment [Line Items]      
Leases amortization expenses $ 12,800 $ 13,400